The tappet in an internal combustion engine is a well-known device, and is also commonly referred to as a lifter, valve lifter, or tappet. For examples of common forms of tappets, see William H. Crouse and Donald L. Anglin's Automotive Mechanics (McGraw-Hill 10th Edition), ISBN 0-02-800943-6 at pp. 131, and 169-170 and Smokey Yunick's and Larry Schrieb's Power Secrets (S-A Design Books 1989) ISBN 0-931472-06-7 at pp. 76-80, both of which are incorporated herein by reference.
FIG. 1 depicts a typical tappet application for a push rod engine. In general, a lifter or tappet interacts directly with a rotating cam shaft in the engine's valve train. That interaction begins the chain of events that converts the rotary motion of the camshaft into the reciprocating motion of the engine's intake and exhaust valves. The amount of horsepower generated by an engine is related to how efficiently the valve train operates. Indeed, it is common knowledge that, of all the adjustments that can be made to an internal combustion engine, adjustments to the valve train have the greatest impact on increasing horsepower.
In general, the more efficiently air enters and combusted gas exits an engine, as controlled by the opening and closing of the intake and exhaust valves, the more horsepower the engine will produce. "Lifting," or opening the valves as high and as fast as possible, and closing the valves as fast as possible, are necessary to obtain efficient air and gas flow, and to achieve optimum horsepower. "High lift" is generally obtained by designing a camshaft having aggressive cam lobes with steep flank angles. Consequently, in high-performance applications, a tappet must be able to reliably negotiate the contour of an aggressive cam lobe at extremely high rpm's. In addition, the tappet must be durable and capable of withstanding extreme frictional forces and high valve spring pressures.
Push rod-type internal combustion engines typically use one of four types of tappets or lifters: the flat mechanical tappet, the msuhroom tappet, the roller tappet, or the hydraulic tappet. Each of these types of tappets or lifters is discussed briefly below.
The single piece, flat mechanical tappet is inexpensive, simple to produce, and reliable in stock environments, and has been the industry standard for years. In high performance applications, however, the flat mechanical tappet has several limitations. First, the flat mechanical tappet requires an extensive and detailed break-in procedure. The break-in procedure typically includes: (1) polishing the lifter foot without disturbing the contour of its convex foot; (2) coating the camshaft lobes with a high performance lubricant; (3) preheating the engine oil before starting the engine, (4) installing light weight valve springs; (5) starting and running the engine for about thirty minutes at about 2500 rpm's to ensure that adequate oil circulation is present in the valve train, and that the tappets are broken in slowly; (6) after shutting down the engine, installing the proper valve springs. This tedious process is necessary to obtain optimum performance from both the tappets and camshaft. Second, mechanical tappets do not work well in high performance applications using aggressive camshafts characterized by lobes having steep opening and closing flanks.
The "mushroom tappet" was developed in an effort to address some of the limitations of the standard mechanical tappet, particularly for use with aggressive cam shaft designs. The mushroom tappet uses a foot with a larger diameter than the body of the tappet, which allows it to more easily negotiate the steeper flank angles of aggressively designed cam lobes. However, several drawbacks are also associated with mushroom tappets. First, before a mushroom tappet can be used, the engine block must usually be machined to ensure adequate clearance with the enlarged tappet foot. Second, the enlarged mushroom tappet foot requires that the tappet be inserted and removed from the bottom of the engine block, thereby complicating repairs or maintenance on the valve train. Third, even the mushroom tappet is characterized by relatively high friction rates requiring significant lubrication.
Moreover, all mechanical tappets are designed to rotate in their bore. The rotation is induced when the convex surface of the lifter foot is in contact the tapered, rotating, cam lobe. The rotation of the lifter or tappet in its bore is necessary to avoid prematurely wearing the lifter foot and cam lobe. However, several additional disadvantages are associated with tappet rotation. First, as the lifter rotates, considerable friction is generated between the surface area of the inside diameter of the lifter bore and the surface area of the outside diameter of the lifter body. Thus, mechanical tappets have relatively high friction rates that often require extensive modifications to the engine to increase the oil flow to the cam lobes and upper valve train in high-performance applications. Secondly, because mechanical tappets rotate, the use of "Rev Kits" (discussed below) has not been successful. Third, because the entire mechanical lifter rotates, it is not possible to use an offset push rod cup, which is often needed to gain additional push rod/cylinder head clearance in some applications. More specifically, offset pushrod cups are typically used in applications where the intake ports of the engine block have been bored to a larger size thus sometimes creating a less than zero clearance between teh push-rod and engine block.
To reduce the adverse effects of friction between the tappet foot and cam lobe, it is highly desirable to make mechanical tappets that are both light and strong, thereby reducing friction. However, the tappet must still be strong enough to withstand the extreme pressures exerted from the valve springs and cam lobe, and durable enough to withstand the rotational forces between the cam lobe and cam foot. As a result, many types of light-weight, exotic, and expensive materials have been used to fabricate tappets. The optimum solution is one that would be able to utilize two different metals in the lifter. This would make it possible to use one type of metal for the lifter body, and one type for the lifter foot. However, the typical one piece design of a mechanical tappet dictates using the same material for the entire lifter body.
The "roller tappet" was developed in large part to overcome the many disadvantages of the mechanical tappet. Roller tappets reduce friction between the cam lobe and lifter foot, thereby reducing lubrication requirements. Thus, roller tappets are desirable in high performance applications, as they can maintain valve train stability at high rpm's and aggressive cam shaft designs. However, they likewise have several drawbacks.
First, many racing circuits do not allow the use of roller tappets. For example, one of the world's largest racing circuits, the Winston Cup Series, prohibits the use of roller tappets. Second, to achieve optimum performance with roller tappets, it is necessary to install an anti rotational device and Rev Kit, thereby further increasing the number of valve-train components, as well as the likelihood of failure. If failure occurs in a roller tappet, typically the results are instantly fatal to the engine.
A fourth common form of lifter is a hydraulic lifter. Hydraulic lifters have several advantages over both mechanical lifters and roller lifters. Hydraulic lifters automatically compensate for any clearance changes caused by temperature variation or wear. Thus, they should never need adjustment. Also because there is no clearance between the lifter foot and the cam lobe, hydraulic lifters are extremely quiet while in operation when compared to both mechanical or solid lifters. Mechanical or roller lifters need to have some clearance or "lash" between the lifter foot and the cam lobe to act as a cushion to allow for any tolerance changes due to thermal expansion or contraction encountered during repeated engine cycles.
However, hydraulic lifters also have some undesirable qualities. Hydraulic lifters are only as reliable as the cleanliness of the engine oil. Thus, if any dirt is present in the oil, the lifter will not compress or decompress properly, and valve and camshaft damage would soon result. Hydraulic lifters also do not work well at high rpm's, because the lifters have a tendency to "pump up" as the rpm's increase. In other words, as engine rpm's increase, more oil is introduced into the oil chamber, preventing the lifter from compressing and decompressing, and adversely impacting the stability of the valve train. The result, is a loss of compression and horse power because the valves are held off their seats.
Thus, the need exists for a new form of lifter or tappet that combines the many advantages of the different types of lifters or tappets with little or none of their varied disadvantages. Thus, a need exists for a multiple piece, roller-type mechanical tappet for use in high performance applications and that is effective, reliable, and inexpensive to produce.